Electrolytic refinement of metals



United States Patent deceased Filed Feb. 24, 1958, Ser. No. 717,167 3 Claims. (Cl. 20464) This invention relates to the electrolytic refining of metals by the use of soluble anodes. The present application is a continuation-in-part of my co-pending application, Serial Number 463,069, for US. Letters Patent, filed October 18, 1954, and now abandoned.

It is well known in the metallurgical art to make use of electrolytic processes in the refining of metals. By use of such processes, resultant metals may be cathodically deposited having a very high degree of purity.

Electrolytic refining takes place in a cell having three essential parts, each performing a specific function when an electromotive force is applied from an outer source, and these are the anode, the electrolyte, and the cathode. In the process, ions of the metal whose production is desired are released from an impure anode to the electrolyte. The ions then migrate through the electrolyte to the cathode where they are electrically discharged in the form of pure metal. The composition of the anode material must be such that the metal whose preferential solution at the anode and deposition at the cathode is sought, must be less noble (i.e. require a lower electrical potential for ionization than a more noble element) than other constituents or impurities of the anode material. Itis this characteristic of composition which makes possible electrolytic purification, for by controlling the anode potential, components in the anode may be either selectively ionized or left in the anode.

In the course of electrolysis, the components of the anode which are not ionized remain at the surface of the anode and may cause chemical polarization and attendant increased difiiculty in the control of the process. Often such an accumulation of insoluble elements or impurities in the solid form sets a limit for anodic current density permissible for preferential solution of the desired element.

In the past efforts have been made to minimize such polarization by avoiding the use of solid soluble anodes except in cases where the amount of insoluble constituents is small. It is preferred to use molten soluble anodes, because the noble constituents diffuse readily from the anode surface into the bulk of motlen metal, and the polarization is slowed down, but is not vw'th such anodes entirely prevented.

It is the primary object of this invention to provide a solid soluble anode for use in electrolytic refinement processes whose surface will be self clearing, and will thus not be susceptible to the advent of polarization. By such provision it will normally not be necessary to interrupt electrolytic processes on account of the anodes for reasons other than replacement of depleted anodes. Furthermore, it will 'be possible to use high anodic current denisties and thus increase the rate of solution of the desired element from the anode. The manner in which this object is to be carried out is by using combinations of nobler elements with the element to be refined as the anode, which anode has the characteristic of changing on its surface at constant temperature from solid to liquid phase after the major part of the metal to be refined has been anodically dissolved of a portion of the element. If care is taken to select alloy compositions in which the liquid phase is heavier than the electrolyte, the liquid phase will sink to the bottom of the cell thus progressively clearing the surface of the anode.

It is contemplated with-in the present invention that combinations of elements suitable as anodes in the electrolytic refining of high melting reactive metals be provided. High-melting reactive metals are those metals whose compounds with non-metallic elements, as oxygen, sulfur, nitrogen and halogens, have low free energy of formation, and whose melting points are higher than 1000 C. Among the high melting reactive metals which can be refined according to the present invention are titanium, zirconium, hafnium, thorium, columbium, uranium, beryllium, as well as others. When these elements are alloyed with less reactive, i.e. nobler, elements having a low melting point, such as lead, tin or bismuth, alloys can be obtained which will function as soluble anodes according to the present invention.

A further object herein is to provide for the electrolytic refinement of metals by using molten salt electrolytes.

A further object herein is to provide an electrolytic refining process particularly suitable for use in the refinement of high melting reactive metals, which can be made continuous. This object is implemented by providing a process in which impurities will collect in liquid phase at the bottom the cell, and not contaminate the electrolyte. The liquid phase at the bottom of the cell can then be removed without interrupting operation.

How these and many other objects are to be implemented can be best understood-by reference to the accompanying drawings in which:

FIG. 1 shows the phase diagram for the titanium lead system, portions of such diagram being empirical; and

FIG. 2 shows a part only of the phase diagram for the titanium zinc system.

A phase diagram may be defined as a graphical representation of the equilibrium temperature and composition limits of phase field and phase reactions in an alloy system. While FIG. 1 shows a phase diagram for a binary system, phase diagrams for ternary and more complex systems are known. Phase diagrams result from plotting temperature against composition based upon the observed behavior of the alloy being investigated. Phase diagrams are for many alloy systems readily available in standard reference works. For other systems, diagrams have not been worked out, although this may be done in known manner by one skilled in the art when interest arises. The phase diagram for titanium lead shown in FIG. 1 is presented since it is representative for systems of interest in applying my invention involving high melting reactive metals and nobler elements.

In FIG. 1 the left ordinate 10 and right ordinate 11 represent temperature, here given in degrees centigrade, and the abscissa 12 represents composition, here stated in terms of percentage of lead. The lines within the diagram itself indicate phase boundaries, at different temperatures and compositions. The point 13 on ordinate 10 is the melting point of titanium, and point 14 on ordinate 11 is the melting point of lead. Vertical line 15 denotes a composition at which a compound of titanium and lead forms having formula Ti Pb which composition contains approximately 52% lead. Vertical line 16 denotes a composition at which another compound of titanium and lead forms having formula Ti Pb, which composition contains approximately 68% lead. One skilled in the art will recognize from the phase diagram that for all compositions at temperatures within field 17, bounded by lines 18, 19, and 20; field 24, bounded by lines 20, 15, 25, and 26; field 27, bounded by lines 26, 16, and 28; and so much of field 29 bounded by lines 16 and line 30 as is above point 14, the melting point of lead'; solid phase will be in equilibrium with liquid phase material. For compositions at temperatures within the entire field above lines 18, 25, 28, and 30 the alloy will be entirely liquid, and for compositions at temperatures below lines 19, the portion of line 20 to the left of line 15, and line 26, the alloy will be entirely solid, though different solid phases may be present.

In carrying out refinement of metal according to my invention I use a solid anode, and carry out my process at substantially constant temperature. Because ions of the metal to be refined are removed from the anode surface during electrolysis, the composition at the surface obviously changes, with percent of the nobler element increasing. In the case of titanium lead, from an examination of the phase diagram of FIG. 1, it will be seen that for any composition originally in the solid state, with constant temperature an increase of the percent of lead will ultimately result in the formation of liquid phase, and the position of the various lines of the phase diagram indicates the expected composition of liquid phase form. More concretely by reference to FIG. 1, assume an initial anode composition of 60% lead and an operating temperature at 800 C., which would be plotted at point 31 in FIG. 1. During electrolysis, titanium will be removed from the anode surface so that the percent of lead will increase at the surface. The anode will remain solid until approximately 68% lead is present. As the percent of lead then increases, liquid phase will be formed. The composition of liquid phase formed can be determined from the phase diagram by extending horizontal line 32 from point 31. Line 32 intersects line 30 at point 33, corresponding to a composition in the vicinity of 89% lead, and 11% titanium. Thus it is apparent that liquid formed will contain a comparatively small quantity of titanium.

In carrying on electrolytic refinement of titanium using titanium lead anode alloy according to my invention, bearing in mind the objective of obtaining liquid with a composition having a small percentage of titanium, I would preferentially use temperatures below that of line 26, approximately 1200" C., so that the compositional liquid will be determined by reference to line 30. Since I require an initially solid anode at the operating temperature, it will be apparent from the phase diagram of FIG. 1 for titanium lead that the initial anode com position will have to contain less than approximately 68% lead. Furthermore, from the phase diagram it will be apparent that since I require that the anode liquefy as the percent of lead increases at constant temperature, the minimum operating temperature is the melting point of lead.

In FIG. 2 is shown a portion only of the phase diagram for titanium zinc. Again left and right ordinates 36 and 37 represent temperature and the abscissa 38 represents the percent of zinc. Vertical lines 39, 40, and 41 represent compounds having formulae TiZn, TiZn and TiZn respectively. For compositions at temperatures within the field 42, which is incompletely delineated in FIG. 2, liquid and solid phase alloy will be an equilibrium. Field 42 is bounded by lines 41, 43, and 44. For alloy compositions, at temperatures above line 33, the alloy is entirely in liquid phase. Since phase relationships are only shown in FIG. 2 up to temperatures of 1000 C., so that field 42 is not completely delineated, it can only be said that for alloy compositions having less than about 80.5% Zinc at temperatures below 1000 C., such alloy compositions will be solid. However, one skilled in the art would anticipate that alloy compositions having less than 80.5% zinc would be solid above 1000 C. to a temperature at approximately where experiment would ultimately determine that lines 41 and 43 meet.

The field 45 in FIG. 2 could be used in calculating the maximum permissible percent of zinc in an anode in the electrolytic refinement of titanium according to my invention, but it will be observed that this field exists only at temperatures between approximately 425 C. and 500 C. As a practical matter, this temperature range would impose too great a requirement for temperature control in a commercial process, and also I find that other factors, such as melting temperatures of appropriate molten salt electrolytes suggest the advisability of using temperatures higher than 500 C. in carrying out re-' finement according to my invention. For these reasons, the field 42 is of primary interest in carrying out my in-' vention where anodes of the alloy titanium zinc are: used. Consequently, the anode to remain solid wouldhave to contain less zinc than approximately 80.5%.- An extrapolation of line 43 suggests that up to a temperature in the vicinity of 1200 C. such compositions would remain solid and liquefication of such compo' sitions with increase in zinc content could occur as low' as approximately 500 C., determined by line 44.

While the foregoing discussion makes it abundantly clear how permissible operating temperatures, and maxi mum content of nobler element in the anode material, are determined, I have found as a practical matter in the case of high melting reactive metals, that an operating temperature in the approximate range 700 C. to 1000" C. is generally most suitable. With this range of openating temperatures, Table I sets forth examples of maxi-' mum permissible percentages of nobler element in binary alloys in which the high melting reactive metal constitu cut is refinable according to any invention? As is the case for titanium-lead and titanium-zinc, the values for maximum permissible percentage of the nobler element for the other systems given in Table I are like-- wise apparent from an examination of the phase dia-- gram for the particular system. Furthermore, this information for systems not mentioned in Table I may be similarly derived from the phase diagram of the particu-- lar system involved. It should be pointed out that for systems containing a high melting reactive metal refinable according to my invention, the phase diagram will, like those for titanium-lead and titanium-zinc, generally show the presence of a plurality of compounds, at least one of which will border a field in the phase diagram indicating that the solid compound is in equilibrium with liquid phase alloy.

It is preferable to use anode alloys with a lower content of the nobler element than indicated in Table I to insure that the anode does not contain portions withliquid phase before electrolysis is started, since otherwise the anode will not retain its shape when immersed in the molten electrolyte at the operating temperature. Generally, I have found that the percentage of nobler elment present should be at least 5% lower than the permissible maximum as provided in Table I.

The foregoing indicates how the maximum permissible percentage of nobler element may be determined. As a theoretical matter, there is no fixed minimum for nobler element content, but in practice it is desirable to have a substantial quantity of nobler element present in the alloy, and one would certainly use more than a trace of nobler element. In the case of titanium, I have found it preferable to use anodes having a minimum of 40%- 50% of the nobler element present.

In order that the mechanisms operating during electrolytic refinement according to by invention may be better understood, the progress of events will now be described with a titanium-lead anode as example. Reference to the titanium-lead phase diagram will aid one skilled in the art in understanding the ensuing description. An electrolytic cell is used containing. a fused salt electrolyte, the electrolysis being carried out at the elevated temperature range approximately of 700 C. to 1000 C. in an inert atmosphere which may be argon or helium. A suitable cathode, such as pure titanium, is present immersed in the electrolyte and being appropriately charged. An initially solid anode, also appropriately charged, is immersed in the molten electrolyte.

The anode will contain less than 63% lead by weight (68% lead permissible maximum less 5% to insure solidity). As is seen from the phase diagram (line 16) at 68% lead, titanium-lead forms a compound which corresponds to the formula Ti Pb. When a solid titaniumlead anode containing more titanium than corresponds to the formula Ti Pb is used initially as the anode in a fused salt electrolyte, titanium dissolves in the form of bivalent ions, and the content of lead in the surface layers of the anode increases with time first to form Ti Pb (a solid compound). On further anodic dissolution of titanium from the compounnd Ti Pb, a liquid solution of titanium in lead will be formed at the operating temperature. From an examination of the titanium-lead phase diagram, one skilled in the art will readily see that for an operating temperature of 1000 C., a liquid titanium lead alloy theoretically containing about 20% titanium will form while at 700 C., a liquid titanium-lead alloy theoretically containing about 10% titanium will form.

The liquid titanium-lead alloy will form at the surface of the anode, since it is from there primarily that electrolysis removes titanium ions. Thus, the surface is enriched as to lead content, and liquid forms at the surface. As the liquid forms, it drops from the anode, and collects at the bottom of the cell. Such liquid can be tapped out of the cell and reused for the preparation of solid titanium-lead anodes with higher titanium content. During electrolysis, the anode tends to maintain in its surface layers the composition of the solid phase corresponding to the formula Ti Pb, which has a higher content of titanium than the liquid titanium-lead alloy collecting at the bottom of the cell. This makes the anode potential constant during electrolysis and permits the use of high anodic current density without causing anodic polarization.

There now follow examples of processes in which high melting reactive metals have been refined:

Example I An electrolytic cell was set up in a closed vessel in which an argon atmosphere was present. The anode had the composition 34% Ti, 62% Sn, and 4% iron (as an impurity), and a cathode of pure titanium was used. An electrolyte consisting of 40% CaCl 40% NaCl and 10% TiCl was present, these salts being molten at the operating temperature of 800 C. which was maintained during the course of the run. An anodic current density of 1500 amps/sq. dm. was applied, and electrolysis permitted to proceed for 24 hours. At the conclusion of this period analyses of the electrolyte and of the cathode were made. No tin and no iron were found in the electrolyte, nor was any present in the cathode deposit, supporting the theory that the tin and iron being more noble than titanium will not dissolve anodically. Liquid metal did collect at the bottom of the vessel and analysis showed that this contained tin and iron as the major constituents, with 8% Ti present.

trolyte was fused salt containing 32% zirconium dichloride and 68% sodium chloride. The cathode was made of zirconium. The electrolysis was carried out at 850 C. Under argon atmosphere. The anodic current density was 1000 amperes per square foot. The liquid alloy which dripped from the surface of the solid anode and collected at the bottom of the cell had the composition: 93% tin and 7% zirconium. The cathodic zirconium deposit did not contain tin.

While the preceding has dealt only with binary alloys, my invention is not limited to such alloys. Alloys of high melting reactive metal with two or more nobler elements can be used as solu'ble anodes providing that the alloy is solid at the operating temperature of electrolysis and is converted on its surface to a liquid alloy when the high melting reactive metal is anodically dissolved. Thus, for instance, ternary alloys of titanium with copper and silicon can be used in the electrolytic refining of titanium.

Example Ill Ternary titanium-copper-silicon alloy was used as anode. The alloy contained 50% titanium, 45% copper and 5% silicon. The electrolyte was a fused salt containing 20% titanium dischloride and sodium chloride. The cathode was made of titanium. The electrolysis was carried out under argon atmosphere at 870 C. The anodie current density was within the range from 500 to 800 amperes per square foot. Analysis of the cathodic deposit showed that no silicon or copper was deposited with the titanium. The liquid alloy which dripped from the solid anode during electrolysis and collected at the bottom of the cell contained 12.5% titanium, 79.5% copper, and 8% silicon.

I claim:

1. A process for the electrolytic refinement of a metal to be refined, said metal to be refined being selected from the group consisting of titanium, zirconium, hafnium, thorium, columbium, uranium, and beryllium, comprising the immersion in a molten halide salt electrolyte of a solid alloy anode consisting essentially of said metal to be refined and an element more noble than said metal to be refined selected from the group consisting of lead, tin, bismuth and zinc, said process being carried out in the temperature range 700 C. to 1000 C., a cathode also being immersed in said electrolyte, the application of an electromotive force to said anode, whereupon ions of said metal to be refined are released from said anode to Said electrolyte, the conversion of said anode to liquid state at the surface thereof as ions of said metal to be refined are so released, the removal of liquid state anode material from the surface of said anode, and the deposition of metal to be refined upon said cathode, said process being carried on in an inert gas atmosphere.

2. A process for the electrolytic refinement of titanium comprising the immersion in a molten halide salt electrolyte of a solid alloy anode consisting essentially of titanium and an element more noble than titanium selected from the group consisting of lead, tin, bismuth and zinc, said process 'being carried out at a substantially constant temperature Within the range 700 C. to 1000 C., a cathode also being immersed in said electrolyte, the application of an electromotive force to said anode, whereupon titanium ions are released from said anode to said electrolyte, the conversion of said anode to liquid state at the surface thereof as titanium ions are so released, the removal of liquid state anode material from the surface of said anode, and the deposition of titanium upon said cathode, said process being carried on in an inert gas atmosphere.

3. A process for the electrolytic refinement of zirconium comprising the immersion in a molten halide salt electrolyte of a solid alloy anode consisting essentially of zirconium and an element more noble than zirconium selected from the group consisting of lead, tin and bismuth, said process being carried out at a substantially constant temperature within the range 700 C. to 1000 C., a cathode also being immersed in said electrolyte, the application of an electromotive force to said anode whereupon zirconium ions are released from said anode to said electrolyte, the conversion of said anode to liquid state at the surface thereof as zirconium ions are so released, the removal of liquid state anode material from the surface of said anode, and the deposition of zirconium upon said cathode, said process being carried on in an inert gas atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS Burgess Dec. 5, 1933 Schultz et al. Feb. 14, 1956 Smetana et a1. Dec. 3, 1957 Gullett Dec. 24, 1957 Slatin Nov. 18, 1958 

1. A PROCESS FOR THE ELECTROLYTIC REFINEMENT OF A METAL TO BE REFINED, SAID METAL TO BE REFINED BEING SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM, THORIUM, COLUMBIUM, URANIUM, AND BERYLLIUM, COMPRISING THE IMMERSION IN A MOLTEN HALIDE SALT ELECTROLYTE OF A SOLID ALLOY ANODE CONSISTING ESSENTIALLY OF SAID METAL TO BE REFINED AND AN ELEMENT MORE NOBLE THAN SAID METAL TO BE REFINED SELECTED FROM THE GROUP CONSISTING OF LEAD, TIN, BISMUTH AND ZINC, SAID PROCESS BEING CARRIED OUT IN THE TEMPERATURE RANGE 700*C. TO 1000*C., A CATHODE ALSO BEING IMMERSED IN SAID ELECTROLYTE, THE APPLICATION OF AN ELECTROMOTIVE FORCE TO SAID ANODE, WHEREUPON IONS OF SAID METAL TO BE REFINED ARE RELEASED FROM SAID ANODE TO SAID ELECTROLYTE, THE CONVERSION OF SAID ANODE TO LIQUID STATE AT THE SURFACE THEREOF AS IONS OF SAID METAL TO BE REFINED ARE SO RELEASED, THE REMOVAL OF LIQUID STATE ANODE MATERIAL FROM THE SURFACE OF SAID ANODE, AND THE DEPOSITION OF METAL TO BE REFINED UPON SAID CATHODE, SAID PROCESS BEING CARRIED ON IN AN INERT GAS ATMOSPHERE. 